Current responsive instrument



Sept. 14, 1943. H. F. GRAVE CURRENT RESPONSIVE INSTRUMENT original Filed De. 8, 1957 .sa mnunrer nv rvcLfs PM sfcawo ma F..

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d l u u ai' u u mum IH CYLLES PIN SIeOND Patented Sept. 14, 1943 2,329,528 CURRENT RESPONSIVE INSTRUMENT llans F. Grave,

General Electric Company,

New York Hanover, Germany, assignor to a corporation of Original application December 8, 1937, Serial No.

178.820. 1939, Serial No. ber 14, 1936 3 Claims.

This application is a division of my copending application Serial No. 178,820, iiled December 8,' 1937, on which Patent No. 2,213,086 was granted August 27, 1940.

My invention relates .to current responsive instruments and concerns particularly both-current and voltage-measuring devices.

It is an object of my invention to provide an improved arrangement having high sensitivity for measuring alternating currents and voltages.

It is also an object of my invention to overcome frequency error.

It is still a further object of' my invention to provide an arrangement which is suitable for the use of current responsive elements having relatively high inductance. Other and further objects and advantages will become apparent as the description proceeds.

My invention will be understood more readily from the following detailed description when considered in connection with the accompanying drawing and those features which are believed to be novel and patentable will be pointed out in the claims appended hereto. In the drawing,V .Figure 1 is a graph illustrating the principle of operation of my invention when used in ammeters and in certain classes of voltmeters. Figure 2 is a diagram representing the equivalent circuit of the conventional rectifier instrument connected in theGraetz circuit. Figure 3 is a circuit diagram of one embodiment of my invention. Figure 4 is a circuit diagram of another embodiment of my invention utilizing a pushpull bridge circuit instead of the four-rectifier or Graetz bridge circuit. Figure 5 is a diagram `representing the equivalent circuit of the conven- .i in the Graetz circuit. Figure 6 is a circuitdiagram of an embodiment ofn my""invention V arrangegwas wa voltmeter anfdV Figure 7 is a graph illustratingV the 'principlfof operation of my invention in voltmeters. Fig. 8 is a circuit diagram of a modification in the arrangement of Fig. 4. Like reference characteristics are utilized throughout the drawing to designate like parts.

In view of the fact that the effect of dry rectiers is dependent upon frequency, instruments of the rectifier type equipped with copper oxide or selenium rectiers give indications which vary considerably with variations in frequency. This phenomenon appears to result in part from the capacitance of the rectiiier plates which causes a rectifying effect decreasing with increasing frequency, that is, a negative frequency error. Inasmuch as the rectifier is intended to block vcur- Divided and this application May 29,

276,460. In Germany Decemrent entirely in one direction any effect such as a capacitative effect which permits the current to ow in either direction tends to make the instrument read too low, that is, gives it a negative error.

However, when the galvanometer or instrument i element is one having a considerable inductance the capacitative effect of the dry rectiiiers tends to give the combined instrument a positive error instead of a negative one for the reason that the dry rectiers acting as condensers tend to store up charges which are prevented from flowing through the instrument element except when a current is flowing in a positive direction for the reason that the inductance of the instrument element tends to oppose stoppage of the iiow of 'current therethrough.

This problem arises particularly in case of 'large switchboard and recording instruments having deflecting elements with large numbers of turns and high inductance. If such an instrument arranged in the conventional Graetz circuitl with (dry rectiiiers is connected as an ammeter a frequency characteristic results such as that shown by curve of Figure 1. It will be seen that a positive frequency error occurs.

In accordance with .my invention the error indicated is diminished or overcome by use of an artificial circuit with rectifier measuring instruments containing highly inductive deecting instrument elements. In the case of ammeters the artificial circuit preferably takes the form of a capacitative impedance connected in parallel with the terminals of the usual current responsive unit. The capacitative impedance preferably consists of a condenser and ohmic resistance in series. In the form of the invention shown. in Figure 3 there is a Graetz bridge II consisting oLfourrdry rectifiers I2, I3, I4, and I5 arranged fin two pairsea'c'h of which is connected in series opposition to the end. terminals connected to the input terminals I6 and I1 of the current responsive unit. There is a current responsive instrument element such as the DArsonval galvanometer I8, for` example, connected in the cross circuit of the bridge between the common terminals I9 and 20 of the two pairs of series opposed dry rectiers I2, I3, and I4, I5, respectively. For the purpose of obtaining frequency compensation a capacitative impedance consisting of a condenser 2l and a resistor 22 in series, are connected across the input terminals I6 and I'I in parallel cooperative relationship to the bridge I I. The principle of operation of the embodiment of the invention illustrated in Figure 3 may be the equivalent circuit of the conventional Greetz circuit rectifier instrument represented in Figure 2.

In Figure 2 r represents the pass resistance of the dry rectifiers I2 and I I. C represents' the .back capacitance of the dry rectiners i3 and I4 It will be understood that 7' is V-l and w is 21- times the frequency. For the values of C, L, R usually encountered in practice, Ki increases with increasing frequency within the usual operating range. The curve A represents the percentage error in terms of the indication at a frequency of 50 cycles per second plotted against frequency.

The use of the capacitative impedance in parallel cooperative relationship to the rectifier bridge overcomes the increase in value of Io with the increase in frequency. The corresponding error with the capacitative impedance connected in parallel relationship is shown by curve B in Figure 1.

In Figure 3, I have illustrated the application of my invention to the conventional Graetz bridge circuit but it will be understood that my invention is not limited thereto, for example, in the case oi' a transformer bridge or push-pull circuit as illustrated in Figure 4 the capacitative impedance consisting of the condenser 2| and the resistor 22 is likewise connected in operative shunting relationship to the rectier bridge across one of the windings of the transformer. A

The arrangement of Figure 4 includes a current transformer 23 having a primary winding 24 and a secondary winding 25 with an intermediate tap 2S. A single pair of dry rectiers i4 and i5 connected in series opposition is arranged with the end terminals 2l and 28 connected across the secondary winding 25 of the transformer 23. The

instrument element i8 is connected in a cross circuit between the intermediate points of the bridge in the usual manner, in lthis case between the intermediate point 26 of the transformer winding 25 and the common terminal 2li of the rectiers iti and i5. It will be understood, of course, that the optimum values of capacity and resistance of the circuit elements 2i and 22 will not necessarily be the same in the arrangements of Figures 3 and 4.

In the precise arrangement illustrated in Fig. 4l

the condenser 2i is connected on the secondary side of the transformer 23. The arrangement utilizing a condenser on the primary side 'of the transformer is illustrated in Fig. 8, having a condenser 2i' and a resistor 22' connected in series between the input terminals I6 and il.

If the conventional Graetz circuit instrument is arranged as a voltmeter with a resistance multiplier 29, an equivalent circuit results such as illustrated in Figure 5. In this case the symbols understood more readily from a consideration oi' previously used have the same significance as in connection with Figure 2. In addition the resistance of the multiplier 2! is represented by the symbol S and the applied voltage to be measured is represented by the symbol V. The current ilowing through the moving coil element il is then represented by equation:

If the series resistance S is made so large that the second and third terms in the denominator become negligible with respect to the first term the equation for the voltmeter becomes the equation previously given for the ammeter. 'I'hat is. voltmeters with high resistance multipliers behave like ammeters and the current responsive units in such voltmeters may be compensated in the same manner as described in connection with Figures 3 and 4 namely, by connecting a capacitative impedance in parallel therewith. A corresponding voltmeter arrangement consisting of the current responsive unit of Figure 3 in series with the voltage multiplier 29 is illustrated in Figure 6.

If the series resistance of the voltmeter is made equal to zero instead of being very large the current in the current inductance coil i8 is given by the following equation:

In contra-distinction to K1, Kvo decreases with frequency under the same conditions as above, as shown by curve C in Figure v'7. With the high resistance connected in series, Kv becomes Ki divided by S and an error curve A as illustrated in Figure 7 results. With a smaller resistance in series the curves between A and C result. It is evident, therefore, that the influence of frequency becomes a minimum for `a certain value of S. In this case the error follows curve D. By utilizing suitable values of r, C, L, and R, such a small optimum value of S can be obtained, that the voltage measuring instrument may also be used as an ammeter arranged in shunt without the voltage drop across the shunt resistance becoming inadmissibly high. The numbers appearing at the ends of the curves in Figure 7 represent the relationships for the various curves between the series resistance S and the impedance of the combined current responsive unit at cycles per second.

I have herein shown and particularly described certain embodiments of my invention and certain y methods of operationembraced therein for the purpose of explaining its principle and showing its application but it will be obvious to those skilled in the art that many modifications and variations are possible and I aim, therefore, to cover all such modifications and variations as fall within the scope of my invention which is deiined in the appended claims.

What I claim as new and desire to secure by Letters Patent of the United States is:

l. A frequency compensated alternating current measuring instrument comprising a current transformer having primary and secondary windings, a pair of rectiers connected in series opposition across the secondary winding, said secondin series opposition- Cil minal of the pair of rectiers and a capacitative impedance connected across the primary winding of said transformer.

3. A frequency compensated alternating current measuring instrument comprising a series transformer having a primary winding and a secondary winding with anv intermediate tap, a pair of rectiers connected in series opposition across the secondary winding, an inductive current responsive instrument element connected between said intermediate tap and a common terminal of the pair of impedance connected across ing of said transformer.

the secondary Wind- HAN S F. GRAVE.

rectifiers, and a capacitative 

